A vacuum cleaner typically comprises a main body containing dirt and dust separating apparatus, a cleaner head connected to the main body and having a suction opening, and a motor-driven fan unit for drawing dirt-bearing air through the suction opening and the cleaner head, and into the main body. The suction opening is directed downwardly to face the floor surface to be cleaned. The dirt-bearing air is conveyed to the separating apparatus so that dirt and dust can be separated from the air before the air is expelled to the atmosphere. The separating apparatus can take the form of a filter, a filter bag or, as is known, a cyclonic arrangement.
Vacuum cleaners generally include cylinder, or canister, cleaners, upright cleaners and hand-held cleaners. A cylinder vacuum cleaner includes a main body supported by a set of wheels which is dragged along a floor surface by a hose and wand assembly extending between the main body and the cleaner head. The cleaner head is generally releasably attached to the end of the wand which is remote from the main body. An upright vacuum cleaner typically comprises a main body, a rolling assembly mounted on the main body for maneuvering the vacuum cleaner over a floor surface to be cleaned, and a cleaner head mounted on the main body. In use, a user reclines the main body of the upright vacuum cleaner towards the floor surface, and then sequentially pushes and pulls a handle which is attached to the main body to maneuver the vacuum cleaner over the floor surface.
A driven agitator, usually in the form of a brush bar, is rotatably mounted within a cylindrical brush bar chamber of the cleaner head. The brush bar comprises an elongate cylindrical core bearing bristles which extend radially outward from the core. The suction opening is located at the bottom of the brush bar chamber, and the brush bar is mounted within the chamber so as to protrude by a small extent through the suction opening.
An exhaust port of the brush bar chamber is generally located towards the rear of the brush bar chamber. The exhaust port is usually in the form of a circular or rectangular aperture formed in the brush bar chamber. The exact location of the aperture may be determined by various factors, such as the rotational direction of the brush bar, the position of the motor or turbine relative to the cleaner head, and the space which is available behind and/or above the brush bar chamber for accommodating a duct for conveying a debris-bearing air flow away from the brush bar chamber. Particularly for upright vacuum cleaners, such as the Dyson DC24 vacuum cleaner, the desire to provide a compact vacuum cleaner for a user means that the cleaner head is located as close as possible to the main body, which generally results in the duct extending from an exhaust port located in an upper rear portion of the brush bar chamber and over a motor housing of the cleaner head to the main body.
The brush bar is activated mainly when the vacuum cleaner is used to clean carpeted surfaces. Rotation of the brush bar may be driven by an electric motor powered by a power supply derived from the main body of the cleaner, or by a turbine driven by an air flow passing through or into the cleaner head. The rotation of the brush bar causes the bristles to sweep along the surface of the carpet to be cleaned, agitating both the fibers of the carpet and any debris, such as dust particles, located on the surface of the carpet and/or between fibers of the carpet, and resulting in a significant amount of energy being imparted to these dust particles. With the brush bar rotating in such a direction that the bristles move from the front edge of the suction opening towards the rear edge, the majority of the energized dust particles are swept rearwardly through the suction opening and into the brush bar chamber by the rotating bristles.
The trajectory at which the energized dust particles enter the brush bar chamber depends on a number of factors, such as the rotational speed of the brush bar, the stiffness of the bristles and the penetration of the bristles within the fibers of the carpet, but our studies have shown that the energized dust particles tend to enter the brush bar chamber tangentially to the brush bar and at an acute angle of up to 45° to the plane of the suction opening. As a result, and particularly where the exhaust port is located above the rotational axis of the brush bar, the vast majority of the energized dust particles entering the cleaner head will not be swept directly through the exhaust port. Instead, the energized dust particles perform multiple collisions with the walls of the brush bar chamber, and with the bristles and core of the rotating brush bar. The random nature of these collisions can result in some of the energized dust particles being re-deposited on or within the fibers of the carpet. The other energized dust particles remain within the brush bar chamber until the energy of those energized dust particles has reduced, through the aforementioned collisions, to a level which allows the dust particles to become entrained within the air flow passing through the cleaner head from the suction opening to the exhaust port.
In order to increase the proportion of the energized dust particles which become entrained within the air flow passing through a given cleaner head, the flow rate of the air flow generated by the fan unit may be increased, for example by increasing the rotational speed and/or size of the fan unit. However, this will increase undesirably the energy consumption of the motor driving the fan unit.